During the manufacturing of power transformers the insulation, paper windings as well as insulation made of wood or cellulose, will adopt a certain level of humidity depending on the humidity of the surrounding atmosphere. Normally, the insulation material will adopt a humidity of approximately 5-8 percent when being exposed to free air during the production of the power transformer.
However, under operational conditions, the humidity of the insulation material should be as low as approximately 0.5 percent in order to guarantee the required insulation properties. Therefore, the manufacturing of power transformers includes a drying step, during which the transformer is subjected to vacuum and heat, and, depending on the specific technique being used, a vapor phase treatment or circulated hot air and vacuum, in order to lower the humidity of the electric insulation.
According to prior art, during the drying process of power transformers, the length of the process is defined by measuring the extraction of water during the vacuum treatment. The process is stopped when a predetermined water extraction value (for example expressed in ml water per ton insulation and hour) is reached. This method has the disadvantage of not being a measurement of the real humidity in the insulation, but a measurement of the removed amount of water only. Moreover, should there be a certain part of the insulation that dries more slowly than the major part of the insulation, prior art might not be enough precise and not able to determine the momentary humidity of such a part. Therefore, in order to guarantee that a required level of humidity is reached, an excessive length of vacuum treatment needs to be applied.
WO 2008/026997 discloses a method of determining the moisture content in a part of the insulation of a power transformer, which addresses the above problem. The method comprises the following steps: providing a test body, the moisture content characteristics of which corresponds to the moisture content characteristics of the part of the power transformer insulation, subjecting the test body to an atmosphere corresponding to the one that the part of the power transformer insulation is subjected to, determining the moisture content of the test body by means of measurement of di-electric frequency response thereof, and determining the moisture content of the part of the power transformer insulation upon basis of the determined moisture content of the test body. The measurement is performed by applying voltage plates on opposite sides of the test body and applying a voltage across the test body.